The present invention relates to the antennas used with base stations for radio communication.
The take-off in cellular mobile communications has necessitated the installation of a large number of base stations. Cellular operators may encounter difficulties in finding appropriate sites. Apart from the problem of site availability, there is also the nuisance problem as perceived by the public due to the size and unattractive appearance of base station antennas which, of course, have to be positioned high up and clearly visible for the network effeciency. In certain countries, regulations or taxes have been introduced with a view to restricting the number of these antennas.
Using multi-sector antennas enables the number of base station sites to be reduced for a given coverage (see EP-A-0 802 579). However, due to their directivity and multiplicity, these multi-sector antennas are considerably larger than omnidirectional antennas.
In order to increase the gain in directivity of a base station antenna, an array of radiating elements is used, disposed in a specific manner relative to the wavelength to be transmitted and fed by the same radio signals to which appropriate phase shift and amplitude laws are applied. The greater the gain in directivity sought, the larger the array has to be. The order of magnitude of the size of each radiating element is determined by the wavelength transmitted, i.e. in the decimetric range, and their arrayed arrangement leads to antennas that may be one to several meters in dimension.
The difficulties outlined above are further aggravated by the deployment of networks using different wavelength ranges. In Europe, for example, second generation digital systems use a band near 900 MHz (GSM,  less than  less than Global System for Mobile communications greater than  greater than ) and a band near 1800 MHz (DCS,  less than  less than Digital Cellular System greater than  greater than ), and future third generation systems (UMTS,  less than  less than Universal Mobile Telecommunication System greater than  greater than ) will use a frequency band near 2000 MHz. In order put in place an infrastructure for a new type of network, an operator who is already operating a different type of network has to provide new antennas. Either he will have to secure new sites or he will have to install more antennas on existing sites. In either case, there will be more antennas.
Furthermore, installing antennas operating in frequency ranges whose ratio is a small integer on the same site causes isolation problems due to the reception by one antenna of harmonics of the frequencies transmitted by another antenna. This situation will arise in the case of the GSM and DCS bands, for which, it is considered that the antennas, already cumbersome, must be spaced at least 50 centimeters apart.
A main object of the present invention is to propose an antenna arrangement that will enable radiating elements having different radiation characteristics (in terms of directivity and/or frequency) to be used together in a relatively compact layout in order to limit the difficulties outlined above.
Accordingly, the invention proposes an antenna for a radio communication base station, comprising several primary sources fed independently and arranged to have different radiation characteristics, the primary sources being placed in a first medium so as to be spatially decoupled. According to the invention, the antenna further comprises at least one second medium covering the first medium and having a substantially lower characteristic impedance than the first medium. Each primary source has at least one direction of focus perpendicular to the interface between the first and second medium, along which the distance between said primary source and said interface is substantially equal to xcex1.(2p1xe2x88x921)/4 and the second medium has a thickness substantially equal to xcex2.(2p2xe2x88x921)/4, where xcex1and xcex2 denote the wavelengths radiated by said primary source in the first and second media, respectively, and p1 and p2 are integers.
The media surrounding the primary sources exhibit resonance conditions which procure a gain in directivity, in elevation and optionally in azimuth. The principle of physics underlying this resonance has been described in the case of conformed antennas in the article entitled  less than  less than Gain Enhancement Methods for Printed Circuit Antennas greater than  greater than  by D. R. Jackson et al., IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 9, September 1985, pages 976-987. The gain in amplitude obtained by the first and second media, having characteristic impedances Zc1 and Zc2 respectively, is in the order of 2.Zc1/Zc2.
The characteristic impedance Zc of a medium with a relative dielectric constant ∈r and a relative magnetic permeability xcexcr is given by             Z      c        =                  Z        c0            ·                                    μ            r                                ϵ            r                                ,
where Zc0=120xcfx80. Consequently, the first and second media may have parameters ∈r and xcexcr adapted as a function of the desired gain.
In a preferred embodiment, adaptation will essentially focus on the dielectric constants ∈r, in order to use more readily available materials. Generally speaking, a medium with a high ∈r will be used for the second medium and ∈r≈1 in the first medium so as to maximize the ratio             Z      c1        /          Z      c2        =                                          ϵ            2                    ·                      μ            1                                                ϵ            1                    ·                      μ            2                                ≈                            ϵ          2                          ϵ          1                    
(where ∈r=∈1, xcexcr=xcexc1 in the first medium and ∈r=∈2, xcexcr=xcexc2 in the second medium).
It is also possible to use composite materials, whereby the values of ∈r and/or xcexcr can be adjusted to suit requirements.
In order to further enhance the gain of the antenna, the first medium may be covered by a superposition of focusing layers, the first focusing layer, adjacent to the first medium, being formed by said second medium, and each focusing layer being formed by a medium of a thickness substantially equal to xcexi.(2pixe2x88x921)/4 along the direction of focus of each of the primary sources, where xcexi denotes the wavelength radiated by said primary source in the medium forming said focusing layer and pi is an integer. The i-th focusing layer is formed, for each odd integer i, by a medium having a characteristic impedance substantially lower than the media located on either side of said i-th focusing layer. In particular, the i-th focusing layer may be made up, for each odd integer i, of a medium having a ∈r substantially higher than the media located on either side of this i-th focusing layer.
Increasing the number of focusing layers increases the gain in amplitude, which will be in the order of 2.   2  ·            ∏              m        =        0            k        ⁢                  Z                  c          ⁡                      (                                          2                ⁢                m                            +              1                        )                                      Z                  c          ⁡                      (                          2              ⁢              m                        )                              
if there are 2k focusing layers over the central high impedance medium, and in the order of   2  ·            ∏              m        =        1            k        ⁢                  Z                  c          ⁡                      (                                          2                ⁢                m                            -              1                        )                                      Z                  c          ⁡                      (                          2              ⁢              m                        )                              
if there are 2kxe2x88x921 focusing layers, Zci denoting for ixe2x89xa72 the characteristic impedance of the (i-1)-th focusing layer (see H. Y. Yang et al.,  less than  less than Gain Enhancement Methods for Printed Circuit Antennas through Multiple Superstrates greater than  greater than , IEEE Transactions on Antennas and Propagation, Vol. AP-35, No. 7, July 1987, pages 860-863).
In one embodiment of the antenna according to the invention, the primary sources are fed and arranged to radiate at different wavelengths. The antenna is then adapted to sites where base stations operating in different frequency bands are installed.
The dielectric media may be disposed parallel to an ground plane, in which case the antenna may be fitted on a wall. In another advantageous layout, the primary sources are disposed along an axis about which said media has revolution symmetry. This being the case, it will be possible to make omnidirectional and/or multi-sector antennas of a reduced size.